1. Field of the Invention
The present invention relates generally to the field of scrambling and transmission systems and more specifically, to an external security module for a television signal decoder of a broadcast, satellite, or cable television transmission system. Additionally, the present invention is more specifically directed to a method of transmitting subscriber information to subscription television signal distributors and methods for converting a television signal decoder to accept digital television signals. The present invention has particular application for B-type Multiplexed Analog Component (B-MAC) satellite transmission, but may also be used for NTSC (National Television Standards Committee), PAL, SECAM, or proposed high definition television formats. In addition, the scrambling system of the present invention can be used in applications in related fields such as electronic banking networks, telephone switching systems, cellular telephone networks, computer networks, etc. The system has particular application to so-called "conditional-access" multichannel television systems, where the viewer may have access to several "basic" channels, one or more "premium" or extra-cost channels as well as "pay-per-view" or "impulse pay-per-view" programs.
2. Description of the Relevant Art
In a pay television system, a pay television service provider typically protects the signal from unauthorized subscribers and pirates through scrambling.
For the purposes of the following discussion and this invention, the term "subscriber" means one who is paying for the television service. The "subscriber" could thus be an individual consumer with a decoder in his own home, or could be a system operator such as a local cable TV operator, or a small network operator such as a Hotel/Motel operator with a central decoder for all televisions in the Hotel or Motel. In addition, the "subscriber" could be an industrial user, as described in U.S. Pat. No. 4,866,770 assigned to the same assignee as the present application and incorporated herein by reference.
For the purposes of this invention, a network is defined as a program source, (such as a pay television provider), an encoder, (sometimes called a "headend"), a transmission means (satellite, cable, radio wave, etc.) and a series of decoders used by the subscribers as described above. A system is defined as a program source, an encoder, a transmission means, and a single receiving decoder. The system model is used to describe how an individual decoder in a network interacts with the encoder.
The scrambling process is accomplished via a key which may itself be encrypted. Each subscriber wishing to receive the signal is provided with a decoder having an identification number which is unique to the decoder. The decoder may be individually authorized with a key to descramble the scrambled signal, provided appropriate payments are made for service. Authorization is accomplished by distributing descrambling algorithms which work in combination with the key (and other information) to paying subscribers, and by denying that information to non-subscribers and to all would-be pirates.
The key may be transmitted as a data signal embedded in the normal television transmission associated with the identification number of the decoder. In a typical television signal, there are so-called "vertical blanking intervals" (VBI) occurring in each field and "horizontal blanking intervals" (HBI) occurring in each line between the chrominance and luminance signals. Various other signals can be sent "in-band" in the vertical and horizontal blanking intervals including additional audio channels, data, and teletext messages. The key can be embedded in these "blanking intervals" as is well known in the art. Attention is drawn in U.S. Pat. No. 4,829,569 assigned to the same assignee as the present application and incorporated herein by reference, showing how such data can be embedded in a B-MAC signal. Alternatively, the key may be sent "out-of-band" over a separate data channel or even over a telephone line.
Maintaining security in a conditional-access television network depends on the following requirements:
(i) The signal scrambling techniques must be sufficiently complex to insure that direct encryptographic attack is not practical. PA0 (ii) keys distributed to an authorized decoder cannot be read out and transferred to other decoders. PA0 (i) It must be impossible to read or modify the SSN and key memories in the decoder. PA0 (ii) It must be impossible to observe the key decryption process, or the links between the four elements (207, 208, 212, and 213) of the decoder.
The first condition can be satisfied by practical scrambling algorithms now available suchas the DES (Data Encryption Standard) or related algorithms.
The second condition requires the physical security of certain devices within the television signal decoder and is much more difficult to satisfy. Such a device must prevent observation of both the key decryption process and the partially decrypted key signals.
FIG. 1 shows a prior art conditional-access system for satellite transmission. In encoder 101, the source program information 102 which comprises video signals, audio signals, and data is scrambled in program scrambler 103 using a key from key memory 104. The scrambling techniques used may be any such techniques which are well known in the art. The key can be a signal or code number used in the scrambling process which is also required to "unlock" or descramble the program in program descrambler 108 in decoder 106. In practice, one key can be used (single layer encryption) or more than one key (not shown). The key be usually changed with time (i.e.--monthly to discourage piracy. The scrambled programs and the key are transmitted through satellite link 105, and received by conditional-access decoder 106. Decoder 106 recovers the key from the received signal, stores it in key memory 107 and applies it to program descrambler 108 which descrambles the scrambled program received over satellite link 105, and outputs unscrambled program 109. The system is not totally secure, as the key is transmitted in the clear through the channel and is available for recovery by pirates.
To overcome this difficulty and referring to prior art FIG. 2, a method of protecting the key during distribution is introduced into the system of FIG. 1. Prior to transmission, the key used to scramble source program 202 in program scrambler 203 is recovered from key memory 204 and itself encrypted in key encryptor 210 using a secret serial number (SSN) from serial numbers data base 211 which contains a list of the secret serial numbersof all legitimate subscribers. These secret serial numbers may relate to the unique identification numbers mentioned above for each decoder of a network of such decoders. The source program has now been scrambled using the key, and the key itself has been encrypted using a secret serial number. Thus, the key is not subject to compromise or recovery during transmission in comparison with the system of FIG. 1. In order to descramble the program, the pirate must first obtain the secret serial number of a legitimate decoder, match it with the appropriately encrypted key, decrypt the key, and then descramble the program. The secret serial number is installed in decoder 206, for example, during manufacture in SSN memory 212 resident in decoder 206. The secret serial number is therefore unavailable to pirates provided that decoder 206 remains physically secure.
Each secret serial number is unique to an individual decoder or, at least, unique to a group of decoders in order to be reasonably secure. The encrypted key may therefore be transmitted to each decoder individually by cycling through a database 211, containing all the secret serial numbers of the network in encoder 201 and forming a separate key distribution message in an addressed data packet individually addressed to each authorized decoder in the network. An individual decoder recognizes when its encrypted key has been received by reading the key distribution message attached to the encrypted key. A typical address data packet is depicted in FIG. 9 and described more fully below.
In known B-MAC systems, the key is distributed in an addressed data packet individually addressed to a particular subscriber's decoder by means of its unique identification number. The addressed data packet is typically inserted in lines 4 through 8 of the vertical blanking interval. Each addressed data packet is typically addressed to one individual decoder. As there are sixty fields generated per second (30 frames of 2 interlaced fields each) in a B-MAC or NTSC television signal, at the rate of one addressed data packet per field, a possible sixty different decoders (or groups of decoders) can be addressed each second, or 3600 per minute, 215,000 per hour, and over 5 million per day. Since each decoder need only be addressed when the service level or encryption level changes, there are sufficient frames available to individually address each decoder even in large systems. The address rate of the decoders may be increased by transmitting more than one addressed data packet per field. Additional data packets may be inserted in the vertical blanking interval or in the horizontal blanking intervals of each frame. The total number of possible addressable decoders is a function of the number on data bits available for decoder addresses. The B-MAC format typically uses 28 bits for decoder addresses, allowing for over 268 million possible decoder addresses. Attention is drawn to the United State Advanced Television Systems Committee Report T2/62, "MULTIPLEXED ANALOG COMPONENT TELEVISION BROADCAST SYSTEM PARAMETER SPECIFICATIONS," incorporated herein by reference, which describes the data formation a B-MAC signal.
After receiving the addressed data packet, key decryptor 213 then decrypts the key using the secret serial number stored in SSN memory 212. If service to any decoder 206 in the network is to be terminated, the secret serial number for that decoder is simply deleted from SSN database 211, and decoder 206 is deauthorized at the beginning of the next key period.
In a decoder such as the one shown in FIG. 2, the pay television provider has to rely on the physical security of the decoder box itself to prevent a pirate from reading or modifying the secret series number and key memories in the decoder or observing the key decryption process. In order to provide the necessary physical security, decoder boxes can be equipped with tamper-proof seals, specially headed screws and fasteners, or other tamper resistant packaging to make physical compromise of the decoder difficult. The subscriber is aware that tampering with the decoder could alter the tamper-proof seals or damage the decoder and subsequent examination could lead to discovery.
There are several disadvantages of relying on the physical security of the decoder to maintain system security. First, the pay television provider has to maintain ownership and control over all of the decoders of the network and then rent or lease the decoders to subscribers. The pay television provider is thus responsible for maintenance of all decoders and must maintain an expensive parts inventory and maintenance staff. In addition, in order to initiate service, a serviceperson must make a personal visit to the subscriber's location to install the decoder. In a pay television satellite system, such installation and service calls could be quite costly for remote installations which could be located anywhere in the world. Further, the physical security of a decoder could be breached without fear of discovery if a pirate could obtain a decoder that had been stolen either during the distribution process or from an individual subscriber's home.
Hence, the system of FIG. 2 can be secure only under the followings conditions;
One way to achieve both of these goals is by the use of a so-called "secure microprocessor".